1. Field of the Invention
The present invention relates to a semiconductor light emitting device having a patterned substrate, and a manufacturing method of the same, and more particularly, to a light emitting device having a locally irregular pattern on a growth substrate to prevent a decline in light extraction efficiency resulting from total internal reflection and increase a surface area of an active layer, thereby enhancing external quantum efficiency.
2. Description of the Related Art
A light emitting efficiency of a light emitting device is determined by internal quantum efficiency and extraction efficiency. In general, the internal quantum efficiency is determined chiefly by a structure of an active layer and a quality of an epitaxial film. The extraction efficiency is governed by refractivity of materials or flatness of a surface or an interface. An InAlGaN-based nitride semiconductor has a refractivity of 2.2 to 2.9, and a GaN nitride semiconductor has a refractivity of 2.4. A sapphire employed as a substrate of such semiconductors has a refractivity of 1.78, and an epoxy resin has a refractivity of 1.5.
FIG. 1 illustrates paths of light in a nitride light emitting device.
As shown in FIG. 1, light generated from the nitride light emitting device, when having an incident angle greater than a critical angle due to differences in refractivity, undergoes total internal reflection and thus cannot be extracted outward. That is, at an interface between GaN/air 12 and 13, the light has a critical angle of about 23.6 degrees and subsequently a light extraction efficiency of about 6%. Also, at an interface between GaN/sapphire substrate 12 and 11, the light has an extraction efficiency of about 13%. A pattern may be formed on a surface of the sapphire substrate 11 in order to increase a critical angle at an interface between GaN/sapphire and thus enhance light extraction efficiency.
Meanwhile, nitride semiconductors formed on the sapphire substrate where the pattern is formed have a shape varied by the pattern formed on the sapphire substrate. First, when a pattern with square convex portions repeatedly arranged is formed on a surface of the sapphire substrate, the nitride semiconductors begin to grow from a flat surface of the square convex portions and a bottom surface of the substrate, i.e., c-plane. However, the nitride semiconductors are hardly grown along sides of the sapphire substrate. During a continuous growth, the nitride semiconductors formed on the square convex portions and the bottom surface converge and are gradually grown to define a flat surface.
Meanwhile, when curved convex portions are formed on the surface of the sapphire substrate, the nitride semiconductor begins to grow from the bottom surface whose crystal orientation is a c-plane. The nitride semiconductor does not grow on the curved convex portions of the sapphire substrate due to absence of a c-plane. Therefore, the nitride semiconductor which has started to grow from a flat bottom surface grows vertically and horizontally at the same time. Therefore, during a continuous growth of the nitride semiconductor, the nitride semiconductor fills the curved convex portions to define an overall flat surface.
That is, in a case where the nitride semiconductor light emitting device is manufactured using a patterned substrate, an n-clad layer, an active layer, and a p-clad layer maybe formed before the nitride semiconductor layer grown on the sapphire substrate having a regular pattern formed thereon is completely flattened to thereby form curved indentations in the n-clad layer, active layer, and p-clad layer, respectively. However, the active layer is grown at a different rate and Si- or Mg-doped to a different doping concentration according to a crystal orientation. Therefore, the sapphire substrate having the regular pattern formed thereon as described above renders it hard to manufacture a nitride semiconductor light emitting device with high light emitting efficiency.